1. Field of the Invention
The present invention is related to an exhaust gas purifying apparatus, in particular an exhaust gas purifying apparatus for efficiently purifying PM exhausted from an internal combustion engine such as a diesel.
2. Related Art
Exhaust gas exhausted from an internal combustion engine such as diesel contains particulate matter (hereinafter sometimes referred to as “PM”). Since it is known that PM has an adverse affect on a human body when PM is unpreferably discharged in ambient air, an exhaust gas purifying apparatus in which a particulate filter (hereinafter sometimes referred to as “DPF”) or the like is installed as a filter to trap PM is installed in an exhaust system of a diesel car. PM contained in the exhaust gas is trapped when the exhaust gas passes through the DPF. Thus, PM is deposited in the DPF as time advances, so that PM has to be periodically or continuously removed from the DPF by way of some kind of methods to reproduce the DPF.
There is a general method for removing PM which is described below. In other words, the method is that fuel is regularly injected into the exhaust system (exhaust gas path), the injected fuel is burnt with an oxidation catalyst, the DPF is heated up to around the combustion temperature of PM (about 600 degrees Celsius) with combustion heat generated at this point, and PM deposited in the DPF is burnt in order to be removed.
In addition, as another method for removing PM, a method for shortening the DPF reproduction time as well as facilitating PM combustion by applying a precious metal catalyst onto the inside of the DPF and improving burning spreading as well as the ignitability of the PM is described in Japanese Patent Publication No. 4-57367 (hereinafter referred as to Patent Document 1).
However, there is a problem that both of the abovementioned methods consume fuel to burn PM so that fuel consumption deteriorates.
On the other hand, as contrasted with the method for removing PM by consuming fuel, the exhaust gas purifying apparatus, which is provided with the DPF having a so-called self-reproduction function for enabling it to burn PM by using the heat of exhaust gas and the energy that molecules in exhaust gas have been proposed. The “self-reproduction” herein means that PM deposited in the DPF is removed or reduced by not using external energy such as fuel injection, heater heat, or the like.
For example, the exhaust gas purifying apparatus in which a platinum catalyst as the first catalyst converting NO into NO2 is installed in the up-stream of the exhaust gas path, and an oxidation catalyst as the second catalyst and the DPF are installed in the down-stream of the exhaust gas path is disclosed in Japanese Unexamined Patent Application Publication No. 10-159552 (hereinafter referred as to Patent Document 2). In this exhaust gas purifying apparatus described in Patent Document 2, a reaction in which NO in exhaust gas is oxygenated to generate NO2 (2NO+O2→2NO2, hereinafter sometimes referred to as “first reaction”) occurs by the oxidation catalyst installed in the up-stream of the exhaust gas path. And then, in the down-stream of the exhaust gas path, an oxidation-reduction reaction occurs with NO2 generated by the oxidization of the oxidation catalyst or existing in exhaust gas, and PM (2NO2+C→2NO+CO2, hereinafter sometimes referred to as “second reaction”). By this second reaction, PM is converted to carbon dioxide. Since the energy barrier of the oxidation-reduction reaction with NO2 and PM is lower than that of the oxidation reaction of PM with oxygen (O2), the exhaust gas purifying apparatus described in Patent Document 2 can purify PM at a lower temperature than the temperature used in the method in which PM is burnt with the abovementioned injected fuel.
In the case of the exhaust gas purifying apparatus described in Patent Document 2, PM can be burnt at a lower temperature than the temperature used in the method in which PM is burnt with the injected fuel; however, the catalyst is not activated unless exhaust gas temperature becomes higher than the required temperature (about 350 degrees Celsius). Thus, in the case of the driving state such as idling, low speed driving, and moreover driving with engine braking action in which the exhaust gas temperature is lower than the required temperature, catalyst activity becomes lower so that the reactivity of the first reaction and the second reaction becomes lower. In addition, in the case of the driving state in which NO is less exhausted, the reactivity of the second reaction becomes lower because of the lack of NO. As described above, in the case of the driving state in which the exhaust gas temperature is lower, or NO is less exhausted, there is a problem that PM cannot be removed to clog the DPF.
Alternatively, in Japanese Unexamined Patent Application Publication No. 2002-276338 (hereinafter referred as to Patent Document 3), the exhaust gas purifying apparatus provided with the continuous reproduction type DPF containing an absorbent/occlusion substance (hydrocarbon absorbent material) which absorbs and occludes a reducing agent and oxygen, a precious metal catalyst (hydrocarbon combustion catalyst) and a PM oxidation catalyst (PM combustion catalyst) is disclosed. In the exhaust gas purifying apparatus described in Patent Document 3, the absorbent/occlusion substance is absorbed and occluded when exhaust gas temperature is lower, and the reducing agent and oxygen are detached from the absorbent/occlusion substance when exhaust gas temperature is higher. The detached reducing agent reacts with oxygen by the catalytic action of the precious metal catalyst to be burnt so that combustion heat is generated, and then the PM oxidation catalyst is activated by the combustion heat. By way of the catalytic action of the activated PM oxidation catalyst, the trapped PM is oxidized (burnt) to be removed.
However, in the exhaust gas purifying apparatus provided with the continuous reproduction type DPF, the arrangement of the absorbent/occlusion substance, the PM oxidation catalyst, and the precious metal catalyst is not designed well. For example, when the PM oxidation catalyst is disposed away from the precious metal catalyst, combustion heat generated by reacting the reducing agent detached from the absorbent/occlusion substance with oxygen is not efficiently transmitted to the PM oxidation catalyst. In addition, when the absorbent/occlusion substances are allocated above the precious metal catalyst, hydrocarbon detached from the absorbent/occlusion substance does not contact with the precious metal catalyst so that hydrocarbon is hardly burnt. Furthermore, when the absorbent/occlusion substance is allocated near the PM oxidation catalyst, the absorbent/occlusion substance absorbs combustion heat generated by burning the reducing agent so that the PM oxidation catalyst is hardly heated up.
As mentioned above, in the exhaust gas purifying apparatus described in Patent Document 3, since the absorbent/occlusion substance, the precious metal catalyst and the PM oxidation catalyst are not adequately allocated, combustion heat to activate the PM oxidation catalyst is not efficiently transmitted so that the PM oxidation catalyst is hardly heated up. Therefore, exhaust gas temperature must be increased to remove PM.